1. Field of the Invention
The present invention relates to a method and apparatus for extracting the recording density data of a black color component from a plurality of image data representing, respectively, color component densities for plurality of colors obtained by reading a color image.
2. Description of the Related Art
In digital color image reproducing machines, for example, color copying machines, color facsimile units, color video printers, etc. a color.image is converted by opto-electronic conversion devices into R (red) component density signals, G (green) component density signals and B (blue) component density signals. Such converted signals are digitally converted into image data Dr, Dg and Db for each of the color components. These image data Dr, Dg and Db are converted into recording density data Dy, Dm and Dc for respective recording colors (Y: yellow, M: magenta and C: cyan), respectively and the recording densities for the respective color toners are determined, respectively, based on the recording density data Dy, Dm and Dc.
Theoretically, all the colors can becompletely reproduced by the combination of these data for respective colors C, M and Y. For instance, an achromatic color such as black or grey can be reproduced by synthesizing C, M and Y each at an identical density. However, in the actual case, since the spectral density of each of the toners for recording each of the colors C, M and Y is deviated from the ideal characteristic and since there is a deviation between the theoretical characteristic and the actual characteristic of a color obtained by the synthesis of C, M and Y, it often fails to obtain a desired color only by the use of C, M and Y. For instance, if recording is conducted by synthesizing C, M and Y each by an identical amount for obtaining grey color, chromatic color components are frequently contained in the recorded color.
In view of the above, images have been reproduced in most of recent digital color copying machines with four fundamental colors by adding BK (black) to C, M and Y. In such machine, an achromatic color component is extracted from input image signals, the signals are applied to a black color recording system and the balance after subtracting the achromatic color component from the input image signals is applied to the recording systems for C, M and Y.
Extraction of the achromatic color component, that is, extraction of black color component is performed by extracting the recording density data Dbk of the black color component from image data Dr, Dg and Db. The applicant of this application has proposed in U.S. Pat. Application Ser. No. 260,277 (filed on Dec. 20, 1988), a method and apparatus for extracting the black color component by judging to which hue area among predetermined three hue areas the hue represented by the image data Dr, Dg and Db belong to, setting parameters of an achromatic color (black color component) in accordance with the thus judged hue area, and obtaining recording density data Dbk showing the black toner recording density from the image data Dr, Dg and Db in accordance with the achromatic color extraction parameters thus set.
In this method for extracting the black color component, since the full black BK toner recording density data are obtained, boundaries of at least the following three hue areas have to be disposed.
Hue area recorded by two, i.e., C and BK toners,
hue area recorded by two, i.e., M and BK toners, and
hue area recorded by two, i.e., Y and BK toners.
Thus, there is a problem that a portion for the boundary of the hue areas can not optionally be selected. In view of the above, upon judging the hue area, since it is necessary to form hue signals from the image data Dr, Dg and Db and compare them with signals representing the boundary for the hue areas to thereby judge the hue area, a complicated processing circuit is required for the judgement of the hue area and it is difficult to obtain the apparatus at a reduced cost.
In addition, an optimum result can not always be obtained by recording all of the achromatic color components in the input images by the black toner, but it is sometime preferred to combine with an achromatic color by the synthetic recording of C, M and Y. In view of the above, there has been proposed a method of adjusting the ratio between the achromatic color by the synthetic recording for C, M and Y and the achromatic color of the black toner to a predetermined value by means of a translation table using a ROM.
However, the optimum value for the ratio varies depending on the saturation of the images. It is preferred, for example, to make the ratio for the black toner lower if the color of the image has high saturation, while to make the ratio for the black toner higher if it is near an achromatic color. Therefore, satisfactory extraction for the black color component can not always be conducted for the images of all of saturations by the method of adjusting the ratio to a constant value as described above.